nil and Booleans

nil

nil means "no value". Any variable or table field that was never set is nil:

local x
print(x)

local user = {name = "Ada"}
print(user.name)
print(user.email)

nil
Ada
nil

Reading a missing field isn't an error — it gives nil. Setting a field to nil removes it.

Only nil and false are false

In conditions, only nil and false count as false. Everything else — including 0 and the empty string — is true:

local values = {0, "", "text", false}
for i = 1, 4 do
  local v = values[i]
  if v then
    print(string.format("%q is truthy", tostring(v)))
  else
    print(string.format("%q is falsy", tostring(v)))
  end
end
print(nil and "yes" or "no")

"0" is truthy
"" is truthy
"text" is truthy
"false" is falsy
no

This surprises people coming from Python or JavaScript, where 0 and "" are false. In Lua, test explicitly: if count > 0, if name ~= "".

and / or return values

and and or return one of their operands, not just true/false:

print(nil or "default")
print("given" or "default")
print(false and "x")
print(1 and 2)

local name = nil
local display = name or "Guest"
print(display)

local age = 20
print(age >= 18 and "adult" or "minor")

default
given
false
2
Guest
adult

• x or default — the standard way to give a default value.
• cond and a or b — Lua's version of a ternary operator. It fails if a is nil or false (then you always get b), so use an if in that case.